The present invention relates to a method of manufacturing a pyroelectric or piezoelectric element consisting of a thin film of polyvinylidene fluoride of a vinylidene-fluoride copolymer (hereafter we refer to as PVDF).
Stretched PVDF having the .beta.-Form crystal exhibits a large piezoelectricity and pyroelectricity useful for application in a piezoelectric element or pyroelectric element. Among their applications, a method of manufacturing a pyroelectric element or piezoelectric element from thin films of PVDF is widely desired because the thin film allows application of the device for an ultra sonic device; in the case of the piezoelectric devices to improve the sensitivity, response velocity, and to improve resolving power of the device in the case of the pyroelectric element.
Heretofore, the investigations of stretching of PVDF were limited chiefly to extruded films and partly to press molded films or films cast from a solution. However, the purpose of the investigations was to improve the piezoelectricity and pyroelectricity and investigation for providing thin PVDF films stretched in a high draw ratio was not performed. The reason for this is that the lower limit of the thickness of the extruded film having a uniform thickness is 10 to 20 micron. Further stretching of the thin extruded films in a high draw ratio is difficult because the orientation of the extruded films increases with decreasing thickness. On the other hand, it is necessary to provide a stretching ratio of more than 3 times and preferably more than 3.5 times for piezo and pyroelectricity.
From the reasons described above the thickness of the stretched PVDF film conventionally used was more than 10 micron. It is necessarily a highly technical procedure for manufacturing a stretched film having a thickness of less than 7 micron. Moreover, a very thin stretched PVDF film having thickness less than 7 micron especially less than 3 micron has a tendency to collect electric charges, therefore handling of the stretched film becomes very difficult.
For making piezoelectric or pyroelectric elements, it is necessary to attach an electrode to both surfaces of the stretched PVDF film, and add heat treatment and polarizing. Usually, the electrodes are formed by evaporation. In this process, defects might occur due to the small capacity of heat in the case of thin film. Moreover, the thin film might break or produce creases during the heat treatment or polarizing.